Refresh AGENTS.md so repo workflow guidance does not encode stale release facts

Summary

AGENTS.md exists, but its source-of-truth section is stale enough to misroute product and worker behavior.

Evidence

• /AGENTS.md still says Current public line: 1.5.1.
• /AGENTS.md still says Active release branch in this workspace: release/1.6.0.
• Live repo state now has default branch develop, active release branch work on release/1.7.0, beta tag 1.7.0.beta.1, and stable public line 1.6.0.

Why this matters

PO and worker threads are expected to rehydrate live state first, but the repo-local guardrail doc should not start from incorrect branch and release assumptions.

Scope

• Refresh AGENTS.md source-of-truth guidance so it does not hard-code stale branch/public-line facts.
• Make the doc explicitly tell agents to verify live repo, release branch, GitHub release, and WordPress.org state before acting.
• Keep repo workflow guidance durable without encoding time-sensitive facts that drift quickly.

Acceptance criteria

• AGENTS.md no longer states stale public-line or active-release-branch values as fixed facts.
• The doc clearly distinguishes durable rules from live state that must be rechecked.
• Release-branch, PR-base, and public-version guidance matches current Perform workflow.